/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/Graph/MovieGraphPipeline.h
Writing images to disk is an async process. When you start writing, declare a future with the filename you will eventually write to, and complete the future once it is on disk.
void AddOutputFuture ( [TFuture](API\Runtime\Core\Async\TFuture)< bool > && InOutputFuture, const [UE::MovieGraph::FMovieGraphOutputFutureData](API\Plugins\MovieRenderPipelineCore\Graph\FMovieGraphOutputFutureData) & InData )